The present invention generally relates to hoses for carrying heated liquids, such as hot melt adhesives.
Various processes involve the transmission of a heated liquid a liquid dispensing device to a desired location such as onto a substrate. For example, hot melt adhesives are frequently applied to products and product packaging. Generally, the hot melt adhesives are applied by a dispensing gun which is connected via a hose to a supply tank. Hot melt adhesives are generally solid at room temperature and, accordingly, must be heated prior to their application so that they can flow from the supply tank, through the hose, and to the dispensing gun. To melt and subsequently maintain the hot melt adhesive at the appropriate temperature, components such as the supply tank, the hose, and the dispensing gun are selectively heated by individual heating devices associated with each component. To monitor the temperature of the hot melt adhesive throughout the application process, each component further includes some form of temperature sensing device which is associated with at least one of the heating devices. A controller operates the heating device in response to signals from the temperature sensing device to maintain the hot melt adhesive within a predetermined temperature range.
In heated hoses such as the one disclosed in U.S. Pat. No. 4,455,474, the inner tube of the hose is typically made from a polymeric material such as polytetraflouroethylene (PTFE) or Teflon(copyright) because of its ability to withstand the relatively high temperatures of the hot melt adhesive pumped through the hose. U.S. Pat. No. 4,455,474 is assigned to Nordson Corporation, Westlake, Ohio, the same assignee as the present invention and its disclosure is hereby incorporated fully herein by reference. As PTFE will not withstand the relatively high pressures at which the hot melt adhesive is pumped, i.e., on the order of 200 psi, the PTFE tube is typically encased within a stainless steel braiding. The stainless steel braiding generally has a relatively rough outer surface. Thus, any component wound around and directly contacting the stainless steel braiding may experience abrasion as the entire hose flexes and the wound component moves relative to the stainless steel braiding.
To minimize the risk of abrasion by the stainless steel braiding, the stainless steel braiding is wrapped with at least two layers of a protective tape, such as silicon tape. The silicon tape provides a relatively smooth surface over which other operative elements of the hose, such as heating and temperature sensing devices, can be wrapped without experiencing appreciable abrasion. For example, the heating device is often an electrical resistance heater comprised of two or more bare electrically conductive resistance heating wires collectively wrapped by a layer of tape, such as Kapton(copyright) tape. The Kapton(copyright) tape serves primarily to consolidate the individual bare wires into a unitary group and this unitary group is then wrapped around the layers of silicon tape. In this instance, the silicon tape over the stainless steel braiding is essential because the Kapton(copyright) tape is not abrasion resistant. Without the protective layer of silicon tape, the stainless steel braiding would quickly abrade the Kapton(copyright) tape, allowing the bare wires to contact the stainless steel braiding and causing the electrical resistance heater to short out. Unfortunately, once the heating device shorts out, replacement of the entire hose is generally necessary.
Using a protective tape between the stainless steel braiding and the heating device has several drawbacks. One drawback is the expense and time required to place the protective tape over the stainless steel braiding. Another drawback is that the protective tape acts as a thermal insulation layer between the heating device and the hot melt adhesive. Furthermore, the addition of the protective layers increases the diameter of the hose and may affect its flexibility.
In light of the drawbacks discussed above, it would be desirable to provide a heated hose that includes an protective cover or casing that is not susceptible to abrasion when placed in contact with a reinforcing layer such as a stainless steel braided cover. Advantageously, the normally present protective tape should be eliminated resulting in reduced manufacturing costs and energy requirements.
The present invention is directed to a hose which overcomes the drawbacks associated with previous hoses connecting supply tanks to adhesive dispensing guns. In the preferred embodiment, the hose includes a tube for carrying liquid therethrough. Preferably, the tube is made from polytetraflouroethylene. The tube is encased in a reinforcing layer which has an outer surface. Preferably, the reinforcing layer is a metallic braided cover and more preferably a stainless steel braided cover. A heater wire, which has an inner core contacted in surrounding relation by a protective casing, is wound around the reinforcing layer such that the protective casing is in direct contact with the outer surface of the reinforcing layer.
Preferably, the protective casing is made from one or more abrasion resistant materials including self-lubricating polymers such as polytetraflouroethylene (PTFE), polyamides, polyethene, and ultrahigh-molecular-weight polyethylene (UHWMPE) and/or non-self-lubricating polymers such as acetals. A thermal insulation layer is disposed around the heater wire and the reinforcing layer to act as a barrier to heat loss from the hose.
Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings.